As a toner for use in electrophotographic copiers or printers, a positively charged toner is used. As a charge controlling agent in this positively charged toner, one may use nigrosine dye, triphenylmethane dye, quaternary ammonium salt, and so on. However, these positive-charge-imparting charge controlling agents comport many problems. For example, quaternary ammonium salt has a low charge-imparting ability, as a result, toner images using it tend to experience fogging and reduced image density as the number of copy sheets multiplies. Triphenylmethane dye tend to cause reduction in charge quantity if used under conditions of a high temperature and a high humidity, and contamination of the fixing roll occurs due to the fact that the pigment in the toner tends to bleed on the surface.
Nigrosine has high positive-charge-imparting properties, and is an excellent charge controlling agent, but its charging performance, thermal properties and so on are greatly affected by the production method. As a result, problems that impair print quality tend to occur, e.g., dispersibility in toner resin does not stabilize, and fogging, reduced image density, toner scattering and the like occur as copying is repeated.
In order to resolve such problems, an approach has been adopted where nigrosine is modified with high-acid-value resin having an acid value on the order of 130 to 300, and this is added to the positively charged toner as a charge controlling agent. To be sure, nigrosine that has been modified with high-acid-value resin has better dispersibility than non-modified nigrosine in toner resin. However, even with nigrosine that has undergone such treatment, dispersibility in toner resin still does not reach a level that is satisfactory in practical terms, and further improvement is called for.
Nigrosine is a bluish-black or black colorant, and is manufactured by heating aniline and nitrobenzene to 160° C.-180° C. in the presence of ferric chloride, iron and hydrochloric acid to engender their reaction. After the reaction, the aniline and nitrobenzene are neutralized by adding an alkaline aqueous solution to the reaction liquid, and the aniline layer is fractionated. Subsequently, treatment with alkali is conducted to render it base, and washing, filtration, drying and pulverization processes are executed to manufacture the product.
There is residual aniline in the nigrosine manufactured in this way. In gas chromatography analysis by the fusion method, more than 2,000 ppm of aniline is ordinarily detected (with respect to the analytic conditions, see the section entitled “Best mode for carrying out the invention”). Were the residual aniline in the nigrosine to flow out to the environment, not only would it be hazardous, but if the nigrosine were used as the charge controlling agent of the toner, it would also cause the charging performance of the toner to deteriorate, the fluidity of the toner to decrease, and so on. As aniline in excess of 2,000 ppm has been detected even in conventional resin-modified nigrosine, where the nigrosine is modified with high acid value resin, there is a need not only to improve the aforementioned dispersibility, but also to reduce the amount of residual aniline.
Various studies have been conducted on methods for reducing the amount of this residual aniline. For example, as aniline reduction methods for nigrosine, the following methods, among others, are conceivable (e.g., see patent literature 1):                (1) a method which washes and removes nigrosine by an aniline-soluble solution;        (2) a method which adds a solvent with a boiling point of 100-230° C., and which conducts heating and pressure reduction to remove aniline together with the solvent;        (3) a method which conducts removal by heating and pressure reduction;        (4) a method which conducts removal by heating at normal pressure.        
Furthermore, there is another method where nigrosine manufactured by the normal manufacturing techniques is dispersed in a solvent, oxidation treatment is conducted by adding an oxidizing agent, and steam distillation is conducted (e.g., see patent literature 2). However, it is difficult to dissolve and remove the residual aniline in the nigrosine by the aforementioned method (1), while with the other methods that require heating, the nigrosine undergoes re-aggregation and forms hard particles, as a result, dispersibility in toner resin markedly deteriorates.
Patent literature 1: Japanese Unexamined Patent Application, First Publication No. H6-230611 (25th paragraph)
Patent literature 2: Japanese Unexamined Patent Application, First Publication No. 2002-311652 (24th paragraph)